Pressure swing adsorption gas separation method and apparatus

ABSTRACT

The present invention is a gas separator for separating a gas mixture into a product gas. The gas separator has an adsorbent bed including a separation chamber with first and second ports and a molecular sieve material contained in the separation chamber. A first pumping chamber is connected to the first port. A first valve regulates a flow of the gas mixture between the first port and the first pumping chamber. A first piston is located in the first pumping chamber. A second pumping chamber is connected to the second port. A second valve regulates a flow of the product gas between the second port and the second pumping chamber. A second piston is located in the second pumping chamber. A drive system coordinates operation of the first and second pistons and the first and second valves in a cycle including a pressurization stage, a gas shift stage, and a depressurization stage.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.10/044,791, filed on Jan. 10, 2002, now U.S. Pat. No. 6,511,526, issuedJun. 28, 2003 which claims benefit of Provisional Application No.60/261,630, filed on Jan. 12, 2001, both applications in the names ofTheodore W. Jagger, Alexander E. Van Brunt and Nicholas P. Van Brunt.

BACKGROUND OF THE INVENTION

The present invention relates to the separation of a selected gas from amixture of gases by pressure swing adsorption. The primary goal is tomaximize the ratio of selected gas volume to energy input whileminimizing the ratio of mechanical volume of the separator to theselected gas volume. The present invention separates, from the gasmixture, a concentration of one or more components of a gas mixture fordelivery or storage.

Pressure swing adsorption (PSA) is a frequently used method to separateone component of a gas mixture. For example, pressure swing adsorptionis used to separate concentrated oxygen from air and then deliver it toa patient for medical purposes. A common use in home medical care is thedelivery of 90-95% concentrated oxygen, derived from the atmosphere, atrates up to 6 liters per minute for the treatment of emphysema or otherdiseases of the lungs in the home. The machines used for this purposeare large, bulky, heavy, and require a large amount of power to operate,thus making battery power impractical. Patients must have a supply ofbottled oxygen available when they leave their home because use of anoxygen concentrator outside the home is not convenient or practical. Useof bottled oxygen is undesirable because of its disadvantages: limitedoperating capacity, heavy weight and hazardousness.

PSA is widely used in industrial gas separation processes as well.Industrial PSA processes vary in the type of gas mixture used andselected gas separated.

Generally PSA involves injecting a mixture of gas into a gas separationchamber having an adsorbent bed or molecular sieve bed. One gas isreadily adsorbed when pressurized above atmospheric pressure in theadsorbent bed, while the other gas is less adsorbed. Both of theseparated gases may be utilized or one may preferentially be used whilethe other is vented as waste. The adsorbed gas in the adsorbent bed isreleased upon lowering the gas separation chamber to the originalatmospheric pressure, at such time purging the adsorbent bed. In orderto achieve sufficient concentration of the separated gas, two or moreadsorbent beds are used in either sequential or multi-processing modes.It is common to purge the bed of the adsorbed gas by using a portion ofthe product gas in order to improve the efficiency of the process.

The selected gas in the mixture can be either adsorbed with theremaining mixture, vented to atmosphere, or otherwise removed.Alternatively, the undesirable components may be adsorbed leaving theselected gas to be passed on for storage or immediate use. The adsorbedcomponent is then discharged as a waste gas, stored or utilizedimmediately dependant upon the application.

To further clarify, the following is a description of a typicalcontinuous process.

(1) Feed gas mixture (A+B) into a container with an adsorbent bed, atsome pressure above atmosphere until the bed is saturated.

(2) Gas B must be adsorbed and stopped before it exits the product end.Gas A is moved to temporary or permanent storage from the product end.

(3) Reduce pressure on adsorbent bed.

(4) Extract gas B from the feed end while taking a fraction of gas A andfeeding it back into the product end to purge gas B.

(5) Stop feeding gas A into the product end before it exits from thefeed end.

(6) Return to step 1.

An example of this process is disclosed in U.S. Pat. No. 5,415,683entitled “VACUUM PRESSURE SWING ADSORPTION PROCESS”.

Methods of providing portability, as disclosed in U.S. Pat. No.4,971,609 entitled “PORTABLE OXYGEN CONCENTRATOR” and U.S. Pat. No.4,826,510 entitled “PORTABLE LOW PROFILE DC OXYGEN CONCENTRATOR”,attempt to reduce the physical packaging design and provide on-demandflow, thereby improving efficiency and portability. These designs areseverely limited because they do not improve the inherent low efficiencyof the PSA process. The prior art requires an amount of energyimpractical for sustained battery operation, a small compact size andlightweight apparatus, while still producing the necessary flow rate andproduct gas concentration. Inefficiencies arise in the PSA process fromthe following sources: (a) resistance to gas flow through the adsorbentbed, (b) energy losses in the pressurization/depressurization process,(c) irreversible thermal losses, and (d) inefficiencies in compressors,gas pumps and valves. Negating these inefficiencies while maintainingthe desired flow rate and concentration is required in order to achievea smaller, lightweight overall machine package capable of batteryoperation.

BRIEF SUMMARY OF THE INVENTION

The present invention is a gas separator device using pressure swingadsorption to separate from a gas mixture the concentration of one ormore components of that mixture.

The present invention is a gas separator for separating a gas mixtureinto a product gas. The gas separator has an adsorbent bed including aseparation chamber with first and second ports and a molecular sievematerial contained in the separation chamber. A first pumping chamber isconnected to the first port. A first valve regulates a flow of the gasmixture between the first port and the first pumping chamber. A firstpiston is located in the first pumping chamber. A second pumping chamberis connected to the second port. A second valve regulates a flow of theproduct gas between the second port and the second pumping chamber. Asecond piston is located in the second pumping chamber. A drive systemcoordinates operation of the first and second pistons and the first andsecond valves in a cycle including a pressurization stage, a gas shiftstage, and a depressurization stage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a pressure swing gas separator of the presentinvention.

FIG. 2 is a graph demonstrating the relative timing of pistons, valvesand gas separation of the present invention, during a pressure swingcycle.

FIG. 3 is a table summarizing the functional stages of the gas separatorof the present invention.

FIG. 4 is a graph demonstrating the relative timing of pistondisplacement, valve opening and closure, and pressure relative to eachpiston during the pressure swing cycle.

FIG. 5 is a diagram of product gas and gas mixture in an adsorbent bedduring the pressure swing cycle.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a diagram of pressure swing gas separator 10 of the presentinvention for separating from a gas mixture the concentration of one ormore components of that mixture (product gas). Separator 10 includesadsorbent bed 12, first pump 14, second pump 16, first valve 18, secondvalve 20, controller/energy source 22, a gas mixture inlet 24, firstoutlet 26 and second outlet 28. A gas mixture enters gas separator 10through inlet 24. Waste gas, the gas mixture minus the product gas,exits gas separator 10 at first outlet 26 (feed end). The product gasexits gas separator 10 from second outlet 28 (product end).

Adsorbent bed 12 includes separation chamber 30, which has first port 32and second port 34. A molecular sieve material 36 is contained withinseparation chamber 30, such as an adsorbent pressure swing preferentialmaterial (e.g. zeolite).

First pump 14 includes pumping chamber 40, first piston 42, piston rod44, crank shaft 46, and first drive motor 48. Pumping chamber 40 isconnected to first inlet 24, first outlet 26 and to first port 32 ofadsorbent bed 12.

Second pump 16 includes second pumping chamber 50, second piston 52,second piston rod 54, second crank shaft 56 and second drive motor 58.Second pumping chamber 50 is connected to second outlet 28 and to secondport 34 of adsorbent bed 12.

First valve 18 controls gas flow between first pumping chamber 40 andfirst port 32 of adsorbent bed 12. Second valve 20 controls gas flowbetween second pumping chamber 50 and second port 34 of adsorbent bed12. Controller/energy source 22 operates as a drive system to controloperation of first and second drive motors 48 and 58 (thereby first andsecond pistons 42 and 52) and first and second valves 18 and 20.

The preferred embodiment of the gas separator 10 requires a complextiming sequence of valve opening and closure, piston movement relativeto one another and the valves, and the sizing of the pistons (volumetricdisplacement). All these interactive variables are required for eachproduct gas.

FIG. 2 is a graph demonstrating the relative timing of pistons, valvesand gas separation in the present invention during a pressure swingcycle (and includes a portion of the structure shown in FIG. 1). Axis 60of the graph in FIG. 2 shows degrees/unit of the time and axis 62 showspiston displacement. Adsorbent bed 12, including gas mixture 64 andproduct gas 66 ratios during the pressure swing cycle, is shownlongitudinally along the middle of the graph. The heavy lined sinusoidalwaveforms 68 and 70 show the path of piston displacement for first andsecond pistons 42 and 52, respectively. The actual optimized waveformwould not necessarily be an exact sinusoid, but for clarity this is usedin the drawing. Lines 72 and 74 are used to show the opening and closingof first and second valves 18 and 20, respectively. The dark horizontalsolid lines are used to indicate the closing of the valves while thedashed lines indicate the opening of the valves.

Adsorbent bed 12 is shown elongated along the graph to illustrate whathappens to gas mixture and pistons 42 and 52 go through theirdisplacement cycle. Note in particular the concentration of the productgas in the bed as it relates to the timing of the piston displacementcycle. In this illustration the concentration of the product gas in thebed increases during the pressurization stage, from 0 degrees to about110 degrees. At 110 degrees, the gas shift stage is underway with firstvalve 18 open and first piston 42 moving upward. The pressure in theadsorbent bed is pushing second piston 52 upward during this timehelping to pull the product gas out of the adsorbent bed and recover thepotential energy of the compressed gas. When second valve 20 closes, thedepressurization stage and waste gas removal begins. First piston 42moves downward depressurizing the bed, thereby removing the waste gasfrom the bed and preparing for the entry of a new gas mixture. Toseparate different gases at the required volumetric flow requires newparameters to be selected for volumetric displacement, valve timing andpiston cycles relative to one another.

FIG. 3 is a table summarizing the functional stages of the gasseparator. The stages include a pressurization stage, a gas shift stageand a depressurization stage. A significant difference in this inventionfrom prior art gas separators is the minimization of energy loss atevery stage in order to maximize the ratio of product gas volumeproduced to energy used.

As seen in FIG. 2 and described by FIG. 3, during the pressurizationstage, a gas mixture is pressurized by product gas being compresseddownward into adsorbent bed 12 by downward movement of second piston 52and second valve 20 being open. Meanwhile first valve 18 is closed andupward movement of first piston 42 compresses the gas mixture andincreases pressure of the gas mixture on first valve 18.

During the gas (volume) shift stage, when the gas mixture pressureequals the pressure in adsorbent bed 12, first valve 18 is opened.Because the pressure is the same on both sides of first valve 18, noenergy is expended in the gas flow past first valve 18. After firstvalve 18 is opened, the shift of the gas mixture volume occurs upwardthrough adsorbent bed 12. The product gas moves upward, pushing secondpiston 52 upward and thus recovering much of the energy stored by theprevious movement of second piston 52 downward. At the same time as thegas mixture moves upward, the gas which is selectively adsorbed will beremoved from the gas mixture leaving product gas which adds to theproduct gas already injected in the previous pressurization stage.

There is a critical step in the timing of the stages just before the gasmixture saturates adsorbent bed 12. At this point, second valve 20 isclosed and first valve 18 remains open. If this is not done, part of thegas mixture will enter with the product gas into second chamber 50,reducing the concentration of product gas. During the gas shift stage,second piston 52 has been retracting, aiding in the gas shift andwithdrawing product gas. The closure of second valve 20 does not resultin an energy loss, as the pressure differential across it is zero. Aftersecond valve 20 closes, some product gas can be withdrawn through secondoutlet 28 for use or storage.

During the depressurization stage, adsorbent bed 12 is regenerated.First piston 42 retracts and the pressure in adsorbent bed 12 falls. Theremaining waste gas (gas mixture minus product gas) from the gas mixtureis exhausted out of first outlet 26. The pressure swing cycle thenbegins again with a new pressurization stage.

FIG. 4 is a graph demonstrating the relative timing of gas separator 10.In particular, the graph shows piston displacement, valve opening andclosing, and pressure relative to each piston during the pressure swingcycle. This graph shows displacement of pistons 42 and 52 (axis 76)versus the rotation of crankshafts 46 and 56 (axis 78), in inches anddegrees respectively. FIG. 4 is similar to FIG. 2, with the addition ofshowing the gas pressure found in pumping chambers 40 and 50. Lines 80and 82 show the displacement of first and second pistons 42 and 52respectively. Lines 84 and 86 show the closure and opening of first andsecond valves 18 and 20 respectively. Lines 88 and 90 show the pressurewithin first and second pumping chambers 40 and 50, respectively, duringthe pressure swing cycle.

Engineering refinements can be added to the piston motion, such as, acam drive of the pistons and valves to provide slight pauses in thepistons during cycling, with a pause time to a particular valve shapeand opening speed. These refinements further improve the efficiency, aswould be recognized by one skilled in the art. Thus, changes can be madeto the configuration of this invention, but the proper parameters forthe gas separation process must take into account the critical timinginteraction described.

The pressurization stage begins at a zero degree position of crankshafts 46 and 56 with second valve 20 open. Product gas acts topressurize adsorbent bed 12 as second piston 52 moves downward in acompression stroke. The peak pressure is reached at approximately 140degrees. Simultaneously, first piston 42 is rising upward in acompression stroke pressurizing the gas in first pumping chamber 40while first valve 18 is closed. When the pressure is equal on bothsides, first valve 18 opens. First piston 42 continues to its peakpressure at approximately 175 degrees, lagging second piston 52 by about35 degrees. At this point, while both valves 18 and 20 are open, the gasmixture is shifted upward in adsorbent bed 12. Note that second piston52 aids in the gas shift from about 110 degrees to about 180 degreeswhile it is moving in the same direction as first piston 42.

At approximately 190 degrees, second valve 20 closes and concentratedproduct gas is removed from adsorbent bed 12. At about 200 degrees inthe cycle, first piston 42 is changing direction and thedepressurization stage has begun. At approximately 340 degrees, firstvalve 18 closes and second valve 20 opens, flushing the gas mixturereleased from adsorbent bed 12. A new gas mixture is brought into theadsorbent bed 12 as first piston 42 rises, first valve 18 is closed andsecond valve 20 opens, beginning the pressure swing cycle again.

FIG. 5 is a diagram of gas mixture 64 and product gas 66 in adsorbentbed 12 overtime. FIG. 5 is shown along the same time cycle as FIG. 4.Product gas 66 is indicated by the dense crosshatched area in thediagram. Gas mixture 64 is indicated by the lightly crosshatched area inthe diagram, and may also contain the gas mixture without the productgas 66 moving in and out of the upper layers of adsorbent bed 12. Thelargest volume of product gas 66 is in the bed at around 110 degrees,when the pressure on the gas from second piston 52 is at its greatest.From approximately 200 degrees to approximately 360 degrees product gas66 is extracted and has moved to second pumping chamber 50, as shown bylack of dark crosshatching during this period. At this time adsorbentbed 12 is depressurized and waste gas is moved into first pumpingchamber 40.

The type of gas mixture and the adsorbent bed used must be taken intoaccount as a variable when determining the design parameters. Anotherfactor that has not been mentioned is the effect of the virtual volumecreated by the adsorbent bed. For example, when an adsorbent bed isselective for nitrogen this creates a volume effect that is three timesthe actual volume in the adsorbent bed for the selected adsorbed gas.The effect is slightly less than 2 for the product gas, which in thiscase would be oxygen. Also the effects of the reduced volume for theproduct gas after removal from the gas mixture must be accounted for inthe design. The sizing of the piston displacement for the larger firstpiston 42 compensates for the former factors.

The present invention maximizes the ratio of product gas volume toenergy loss, while minimizing the ratio of mechanical volume of theseparator to the product gas volume. In particular, the desired ratioswere determined by utilization of an adaptive computer program thatmanipulated five variables of a simulated machine as it operated.

The following steps were taken to minimize energy loss and to maximizeproduct gas volume:

(1) Gas flow resistance was decreased by varying the width to lengthratio of the adsorbent bed, as resistance parallel to flow varies as thecube of length.

(2) Energy losses were decreased in the pressurization/depressurizationprocess by converting the potential energy of the compressed gas intousable kinetic energy to move the pistons.

(3) Valve losses of energy were decreased by opening the valves onlywhen the pressure on opposite sides of the valves was equalized.

(4) Losses by temperature changes due to adsorption and desorption weredecreased by reducing the size of the adsorption bed and by runningrapid cycles (typically in the range of about 10 cycles per second) suchthat the average thermal change is approximately zero.

(5) The piston stroke volume was determined based upon the type of gasmixture and adsorbent bed used as well as the product gas volumeproduced.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope or the invention.

What is claimed is:
 1. A method for operating an oxygen concentrator,the oxygen concentrator including an adsorbent bed having a separationchamber with first and second ports, a first pumping chamber connectedto the first port with a first valve, a first piston in the firstpumping chamber, a second pumping chamber connected to the second portwith a second valve, and a second piston in the second pumping chamber,the method comprising: closing the first valve; opening the secondvalve; moving the first piston toward the adsorbent bed; moving thesecond piston toward the adsorbent bed; opening the first valve; movingthe second piston away from the adsorbent bed; closing the second valve;and moving the first piston away from the adsorbent bed.
 2. The methodof claim 1 wherein operation of the oxygen concentrator separates oxygenfrom ambient air.
 3. The method of claim 2 wherein ambient air isintroduced into the adsorbent bed from the first port and oxygen exitsthe adsorbent bed from the second port.
 4. The method of claim 1, andfurther comprising: compressing ambient air within the first pumpingchamber with the first piston; pumping oxygen into the adsorbent bedfrom the second pumping station with the second piston; introducingambient air into the adsorbent bed from the first pumping chamber; andwithdrawing oxygen from the adsorbent bed into the second pumpingchamber.
 5. A method for separating oxygen from ambient air using anadsorbent bed having a feed end and a product end, the methodcomprising: compressing ambient air at a feed end of the adsorbent bedwherein a first pressure is created outside of the adsorbent bed;compressing oxygen into the adsorbent bed from the product end of theadsorbent bed wherein a second pressure is created within the adsorbentbed; equalizing the first pressure and the second pressure such that theambient air moves from the feed end into the adsorbent bed; lowering thepressure within the adsorbent bed wherein the oxygen separates from theambient air; compressing ambient air into the adsorbent bed from thefeed end to push the oxygen from the adsorbent bed through the productend into a product chamber connected to the adsorbent bed at the productend; depressurizing the adsorbent bed; and withdrawing the oxygen fromthe product chamber.
 6. The method of claim 5 wherein a feed chamber isconnected at the feed end with a first piston disposed in the feedchamber and a first valve separating the feed chamber from the adsorbentbed to regulate the flow of ambient air with respect to the adsorbentbed, and a second piston is disposed in the product chamber with asecond valve separating the product chamber from the adsorbent bed toregulate the flow of oxygen with respect to the adsorbent bed.
 7. Themethod of claim 6 wherein the compressing ambient air step furthercomprises moving the first piston toward the adsorbent bed wherein thefirst valve is closed to create the first pressure outside the adsorbentbed.
 8. The method of claim 6 wherein the compressing oxygen into theadsorbent bed step further comprises moving the second piston toward theadsorbent bed wherein the second valve is open to create the secondpressure within the adsorbent bed.
 9. The method of claim 6 wherein whenthe first and second pressures are equalized, the first valve opensthereby allowing ambient air to enter the adsorbent bed.
 10. The methodof claim 6 wherein the lowering the pressure within the adsorbent bedstep further comprises moving the second piston away from the adsorbentbed.
 11. The method of claim 6 wherein the compressing ambient air intothe adsorbent bed step further comprises moving the first piston towardthe adsorbent bed wherein the first valve is open and the second valveis open.
 12. The method of claim 6 wherein the depressurizing stepfurther comprises moving the first and second pistons away from theadsorbent bed.
 13. The method of claim 6 wherein the withdrawing oxygenstep further comprises moving the second piston away from the adsorbentbed wherein the second valve is closed.
 14. An oxygen concentratorcomprising: an adsorbent bed including a separation chamber with firstand second ports and a molecular sieve material contained in theseparation chamber; means for regulating the flow of ambient air withrespect to the adsorbent bed; means for regulating the flow of oxygenwith respect to the adsorbent bed; and means for cycling the oxygenconcentrator through a pressure swing cycle such that oxygen isseparated from ambient air within the adsorbent bed.
 15. The oxygenconcentrator of claim 14 wherein the means for regulating the flow ofambient air comprises a first pumping station connected to the firstport.
 16. The oxygen concentrator of claim 15, and further comprising: afirst piston disposed in the first pumping station; and a first valvelocated between the first pumping station and the adsorbent bed.
 17. Theoxygen concentrator of claim 14 wherein the means for regulating theflow of oxygen comprises a second pumping station connected to thesecond port.
 18. The oxygen concentrator of claim 17, and furthercomprising: a second piston disposed in the second pumping station; anda second valve located between the second pumping station and theadsorbent bed.
 19. The oxygen concentrator of claim 14 wherein the meansfor cycling the oxygen concentrator through a pressure swing cycleincludes a drive system that coordinates operation of the means forregulating flow of ambient air and the means for regulating flow ofoxygen.
 20. A portable oxygen concentrator for separating oxygen fromambient air, the oxygen concentrator comprising: an adsorbent bedincluding a separation chamber with feed end and a product end and amolecular sieve material contained in the separation chamber; a firstpumping station connected to the feed end to regulate flow of ambientair with respect to the adsorbent bed; a second pumping stationconnected to the product end to regulate flow of oxygen with respect tothe adsorbent bed; a drive system to coordinate operation of the firstpumping station and the second pumping station in a pressure swingcycle; and a battery operated power source.
 21. The oxygen concentratorof claim 20 wherein the drive system operates the pressure swing cyclesuch that average thermal change is approximately zero.
 22. The oxygenconcentrator of claim 21 wherein the drive system runs about 10 cyclesper second.